1. Field of the Invention
The invention relates to a resistor element housing PTC characteristics. Such so-called PTC resistors have a resistance which, at a certain switching current density, increases by several orders of magnitude, and are used for current limiting purposes, especially in the event of a short circuit.
The marked increase in the resistance when the switching current density is reached is caused by the fact that, owing to the heating and expansion of the polymer matrix, which are due to increased energy absorption, the embedded conductive particles of the first filler are separated. What was found to be a drawback in this context was that this effect tends to concentrate in a switching zone which, while indeed extending over the cross section of the resistor element, is nevertheless relatively short in the current direction, so that the total voltage drops over a short distance and the predominant proportion of the converted electrical energy is produced in a very small volume. This can easily lead to arcing and damage to the resistor element. Furthermore, this causes a reduction in the sustain voltage of the element, i.e. the voltage said element is able to sustain without too large a leakage current after a short circuit has been broken.
2. Discussion of Background
Inter alia, attempts have been made to improve the relevant behavior of such resistor elements by admixing the material with a second filler having varistor characteristics. U.S. Pat. No. 5 313 184, for example, discloses a congeneric resistor element which includes from 5 to 30% (by vol.) of varistor material in powdered form as a second filler. Expectations regarding an improvement in the voltage sustaining capability of the resistor element were not, however, met in full.